This invention relates to receivers, and in one embodiment, to a dual mode CDMA/AMPS wireless receiver capable of accurately processing CDMA signals in the presence of AMPS interferers.
In the field of telecommunications, Direct Sequence (DS) Code Division Multiple Access (CDMA) transmission is a popular form of communication because a large number of users may communicate over the same frequency band without interference. In DS-CDMA systems, data is transmitted to an intended recipient by initially generating an address corresponding to the data which is to be transmitted. The address is then combined with a unique pseudo-random bit sequence (PN sequence) to form a combined waveform. The pseudo random sequence causes the combined waveform to appear highly uncorrelated, i.e., unpredictable. The combined waveform is then modulated onto a particular carrier frequency, ranging from 1931.25 MHz-1988.75 MHz in the PCS-band systems or in the frequency range of 869.04 MHz-893.97 MHz in the cellular-band systems. Although many users have access to the signals within a particular frequency band (typically 1.23 MHz wide), none can decipher the combined waveform since it appears to be random. Only the intended recipient is able to decode the combined waveform since his receiver alone produces the same PN sequence used to encode the data within the transmitter.
The transmitted data typically consists of two orthogonally-phased data streams (I and Q data) which are interleaved onto a single data stream prior to transmission. The orthogonal orientation between the I and Q data allows the compilation and transmission of the two data streams without interference between them. The modulated data stream is subsequently transmitted as xe2x80x9cchipsxe2x80x9d to the receiver. The receiver removes the modulation and separates the original I and Q data from the single data stream.
FIG. 1 illustrates a typical receiver for the reception of the orthogonal DS-CDMA signals. The CDMA receiver 100 includes an RF amplifier 110, a downconverter 120, an automatic gain control amplifier (AGC) 130, a baseband conversion circuit 140, baseband analog filters 152 and 154, and I and Q channel analog to digital converters (ADCs) 162 and 164. Analog circuitry is shown in white, and digital circuitry is shown in gray.
During reception, A CDMA signal 102 is received by an electromagnetic collecting apparatus such as an antenna (not shown) and supplied to the CDMA receiver 100. The RF amplifier 110, typically a low noise amplifier (LNA), is used to increase the amplitude of the CDMA signal 102. A downconverter 120 is used to convert the input signal 105 to an IF signal 125 of lower frequency which the subsequent circuitry can process. The downconverter may consist of a single or multiple downconversion stages to frequency translate the RF signal to its final IF frequency.
The IF signal 125 is supplied to the AGC circuit 130 which provides variable signal gain to account for the varying distances over which the received signal may propagate. The AGC circuit 130 can be controlled via a gain control signal 132 to provide attenuation or gain in varying degrees, producing an AGC output signal 134. The AGC circuit 130 typically provides a sufficient amount of gain or attenuation so that the amplitude level of the I and Q signals supplied to the analog to digital converters (ADCs) 162 and 164 is within an optimum input power range.
A baseband conversion circuit 140, typically an analog quadrature downconverter circuit, is used to extract the I and Q data from the AGC output signal 134, producing I and Q channel baseband signals 142 and 144. This process typically involves frequency translating the AGC output signal 134 to lower frequency I and Q baseband signals 142 and 144 as well.
The I and Q channel analog filters 152 and 154 are used to filter out any out-of-band signals prior to the ADCs 162 and 164. Additional filtering 127 may be required within the receiver 100 to achieve the necessary out-of-band rejection.
The I and Q analog filters 152 and 154 are also designed to have precise group delay response. Prior to transmission, the phase of the CDMA signal is pre-distorted for optimum signal transmission. In order for the I and Q data to be properly reconstructed at the receiver output, the phase response over the communication channel (i.e., between the transmitter input and the receiver output) should be near linear. Thus, the analog filters 152 and 154 must be designed to provide a specific phase response which, when combined with the filters used within the CDMA transmitter (not shown) is a particular value.
Additionally, the I and Q channel analog filters 152 and 154 must be closely matched to provide substantially identical amplitude and phase responses. The close amplitude and phase matching ensures that the I and Q channel data are equally affected by the filtering stage. The analog filters 152 and 154 are typically realized in switched capacitor form and may be fabricated in IC form or from discrete components.
I and Q channel ADCs 162 and 164 receive the filtered I and Q baseband signals, converting the signals to I and Q channel data 172 and 174, respectively. Two DC offset voltages 175a and 175b are supplied to the ADCs 172 and 174 to correct for the DC voltage level superimposed on the filtered baseband signals 142 and 144 as a side-effect of the downconversion and analog to digital (A-D) conversion process. The I and Q channel data 172 and 174 is then fed into I and Q channel correlators (not shown) to determine the degree of correlation with the receiver""s address code.
One disadvantage of the conventional CDMA receiver is its inability to reject correlated interfering signals. One such type of signal is generated from the Advanced Mobile Telephone System (AMPS), also commonly used in cellular telephony today. The AMPS system is an analog Frequency Division Multiple Access (FDMA) system in which data is communicated using frequency modulation (FM). Each user is allocated a particular carrier bandwidth, typically 30 KHz, which carries that user""s transmissions. The AMPS signals are narrow band (30 KHz) compared to the CDMA signals (1.23 MHz) and are highly correlated.
Unfortunately, the AMPS system transmits its FM signals within the CDMA receiver band, 869.04 MHz-893.97 MHz. When the AMPS and CDMA signals 102 and 104 are both received by a non-linear device, such as the RF amplifier 105 within the CDMA receiver, a two-tone third order intermodulation product or xe2x80x9cAMPS interfererxe2x80x9d can be produced at the amplifier""s output which is within the CDMA receivers band. Once within the CDMA receiver""s band, the AMPS interferer can propagate as a false CDMA signal, causing erroneous data output and signal distortion. The inability of the conventional CDMA receiver to operate in environments where AMPS or other highly correlated signals propagate severely limits the use and operability of the conventional CDMA receiver.
Another disadvantage of the conventional CDMA receiver is the high cost and marginal performance associated with the I and Q channel analog filters 152 and 154. The I and Q channel analog filters 152 and 154 are needed to provide rejection of the out-of-band signals and add phase correction to the incoming signal. Additionally, the I and Q channel analog filters 152 and 154 must be closely matched to each other. If fabricated from discrete components, each of the analog filters 152 and 154 would require an extensive amount of time and labor to tune and test, and represents a significant cost factor of the CDMA receiver.
Alternatively, the analog filters 152 and 154 may be fabricated in integrated circuit (IC) form using Bipolar-CMOS (Bi-CMOS) technology. Bi-CMOS IC processing allows for the fabrication of analog and digital circuitry on the same IC die. Using this IC process, the analog filters can be fabricated in IC form without tuning. However, the Bi-CMOS technology is more expensive to implement and suffers from lower yields compared to the more mature digital CMOS or analog Bipolar IC processing technologies.
A further disadvantage of the conventional CDMA receiver architecture is the implementation of separate I and Q channel ADCs 152 and 154. The I and Q channel ADCs 152 and 154 can have slightly different amplitude and/or phase response characteristics. Implementing separate ADCs increases the likelihood that the I and Q channel data 172 and 174 will exhibit some amplitude and/or phase imbalance.
Still a further disadvantage of the conventional CDMA receiver is the need for DC offset voltage sources 175a and 175b to correct for the DC voltage superimposed onto the IF and baseband signals occurring as a side effect of the downconversion and the A-D conversion process. If left uncorrected, the added DC voltage will result in erroneous data I and Q channel data values and a degraded receiver bit error rate (BER).
What is needed is a new CDMA receiver architecture which remedies the aforementioned shortcomings of the conventional CDMA receiver. Specifically, a new CDMA receiver is needed which can discriminate and remove AMPS interfering signals and other correlated signals from the received signals. Also needed is a CDMA receiver architecture which employs low cost, highly manufacturable filters for providing the required out-of-band signal rejection and phase response. Further needed is a CDMA receiver architecture which consists of a single ADC for converting the IF signal to a digital data stream. Use of a single ADC will reduce the likelihood of introducing amplitude and/or phase imbalance between the I and Q channel data. Lastly, a new CDMA receiver architecture is needed which does not require the use of a DC offset source to correct for an superimposed DC voltage.
The present invention provides for a new CDMA receiver which uses digital noise cancellation circuitry to remove the AMPS interferer or any other highly correlated signal from the receiver during CDMA reception. The new CDMA receiver also employs less expensive, more manufacturable digital IC filters to provide the needed adjacent channel rejection and phase response necessary for proper I and Q data reconstruction. In addition, the new CDMA receiver uses a single ADC to convert the IF signal to a digital data stream containing orthogonally-phase I and Q data. Further, the new CDMA receiver employs IF sampling in the baseband conversion process to remove any superimposed DC voltage from the baseband data.
In one embodiment the CDMA receiver includes a RF amplifier, a downconverter, an analog to digital converter (ADC), a baseband conversion circuit, an I channel digital filter, a Q channel digital filter, an I channel digital noise cancellation circuit, and a Q channel digital noise cancellation circuit. The RF amplifier receives and amplifies and the downconverter downconverts the CDMA signal to an IF signal having an AMPS interfering component. The ADC receives and converts the CDMA+AMPS interfering signal to digital data. The baseband conversion circuit use an IF sampling technique to remove any superimposed DC voltage from the digital data by alternatively negating output data words and subsequently adding data word pairs together, thereby removing any commonly occurring magnitude. The AMPS interfering component is reduced by the noise cancellation circuitry which detects and removes highly correlated signals, such as the AMPS signal.
A further understanding of the nature and advantage of the invention herein may be realized by reference to the remaining portions of the specification and attached drawings.